If the Knudsen number is between 0.01 and 0.5, this is termed Knudsen flow. Because
many process pressures are in the medium vacuum range, this type of flow occurs with corresponding
frequency. Since this is a transitional flow, this range is transited relatively quickly
when pumping down vacuum chambers. This means that the influence of this conductivity
on pump-down times is correspondingly low. It is a complicated endeavor to perform a
precise calculation of conductivity where the flow range is still laminar and yet already molecular,
and this will not be discussed here. A simple approximation for the Knudsen range can
be obtained by adding the laminar and molecular conductivities. Figure 1.7 shows the
conductivities of round, one meter long tubes of differing diameters in all three flow ranges